For Clickpad & Touchpad OS, freely downloadable OS upgrades (containing a boot2) are enough to create a .img suitable for nspire_emu.
However, for the CX and CM, you need a boot1. Even if only a subset of boot1 would be technically required for setting up several ports and loading the boot2 (I mean - for instance, handling serial port communication wouldn't be a required feature for an alternative, emulator-only boot1), the same legal challenges as for distributing official boot1 would get in the way of distributing alternative boot1.